In the energy harvesting technology in which weak environmental energy is converted into electrical energy using electricity generating elements such as photovoltaic cells, thermoelectric generation elements, or piezoelectric elements; the electrical power output by the electricity-generating elements fluctuates in a major way depending on the environmental conditions. In that regard, typically, in order to buffer the fluctuation in electrical power attributed to the environmental conditions, the electricity-generating elements are configured to temporarily store the electrical power in capacitance. Moreover, with the aim of enhancing the electricity storing efficiency and using the stored electrical energy in an efficient manner, the configuration is done to monitor the electricity generating capacity of the electricity generating elements and to monitor the remaining amount of stored electricity in the capacitance.